Fumed silanized and ground silica

ABSTRACT

The invention relates to a hydrophobic fumed silica which is obtained by grinding a fumed silica which, as a result of silanization, has trimethylsilyl groups fixed on the surface; and to coating formulations comprising this silica.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/EP08/055556, filed on May 6, 2008, and claims priority to German Patent Application No. 10 2007 024 35.2, filed on May 22, 2007.

The invention relates to a hydrophobic fumed ground silica, to a process for preparing it and to its use. The present invention further relates to a coating formulation comprising the silica of the invention.

Fumed silica is known from Ullmanns, Enzyklopädie der technischen Chemie, volume 21, page 464 (1982). The fumed silica is prepared by combusting an evaporable silicon compound, for example silicon tetrachloride, in a mixture with hydrogen and oxygen.

The comminution of substances to flours (50-500 μm), powders (5-50 μm) and even greater fineness (less than 5 μm) is common practice in the art. For all comminution tasks, a multitude of technical equipment is supplied and operated, which is matched to the peculiarities of the individual tasks. A good overview of the comminution problems and the various machines is given in Ullmanns Enzyklopädie der technischen Chemie, 3rd edition, volume 1, page 616 to page 638.

In fumed silica, the mean primary particle diameters are considerably lower (5-50 nm) than can be obtained by a mechanical comminution. The primary particles and aggregates of fumed silica with a surface area of 200 m²/g can be visualized in an electron microscope.

The primary particles and aggregates of a fumed silica combine to form larger composites, the agglomerates. In general, the smaller the particle size is or the greater the specific surface area is, and the more highly compacted the fumed silica is, the larger these agglomerates are.

The binding forces with which these agglomerates are held together are relatively weak. Nevertheless, in the incorporation and dissolution of these agglomerates in a liquid system for the purpose of homogeneous distribution of the primary particles and aggregates or low-agglomeration particles, a certain expenditure of shear energy is required. For the dispersion, according to the field of use, a wide variety of different mixing units is used, the crucial factors for the selection being both the viscosity and polarity of the system and the agglomerate strength and the desired homogeneity.

With simple stirrer systems, for example paddle stirrers, direct incorporation of small amounts of silicas usually cannot be carried out satisfactorily, particularly when low-viscosity systems are involved. However, manufacturers of paints and coatings, and also processors, have an interest in achieving an optimal distribution from a performance point of view of the silicas which are used predominantly as thickeners and thixotropic agents with very simple equipment and with a minimum level of time consumption and energy expenditure.

In the case of paddle stirrer dispersion, the coarse silica agglomerates are not comminuted sufficiently and can thus make only a small contribution to raising viscosities and thixotropy. The information is based on a UP resin (unsaturated polyester resin) as a dispersant.

A reduction of the agglomerate size by dispersing outside a liquid system, i.e., in practical terms, under air or by grinding in the conventional sense, was possible in the prior art only to a limited degree, since, in the case of a given agglomeration tendency of the material, the old agglomeration state is soon re-established after the comminution. This effect occurs no later than after recompaction of the material which has been greatly loosened by the mechanical intervention and cannot be shipped or stored in this form. The storage time would also have an effect, in the sense of agglomerate reenlargement.

The mass number and evaluation parameter employed for the state of distribution of a dispersible silica and maximum agglomerate size of the dispersion (granularity) is the so-called grindometer value to DIN 53203.

It is known that fumed silica can be hydrophobized, ground in a pin mill and then classified (US 2004/0110077 A1). This known silica is used as an external additive in toner mixtures.

Hydrophilic fumed silica with a BET surface area of 200 m²/g has a grindometer value, determined in UP resin (Ludopal P6 unsaturated polyester resin from BASF, 2% dispersion) by the DIN method, of 50 to 60 μm.

When this fumed silica is additionally more highly compacted (100 to 120 g/l), the grindometer value is also significantly higher, specifically more than 100 μm, as a result of which an additional, not inconsiderable expenditure of energy is required as a thickener and thixotropic agent.

It is known that a high-dispersity silica with a surface area of approx. 300 m²/g can be ground in a pin mill.

The grindometer value achieved is initially 25 μm for the uncompacted silica.

If this silica is compacted to 50 g/l, the grindometer value rises to 30 μm, and, in the event of further compaction to 75 g/l, even to about 40 μm.

In the course of storage over a period of three months, the ground, unmodified silica compacted to 50 g/l has a grindometer value of 50 to 60 μm.

According to the prior art, reagglomeration can be prevented only if the hydrophilic silica is mixed with 3% by weight of a hydrophobic silica and this mixture is ground by means of an air-jet mill or of a pin mill (EP 0 076 377 B1).

For a fumed silica with a BET surface area of 200 m²/g, even after compaction to 73 g/l or 107 g/l, a grindometer value of 35 μm is attained.

In the case of a fumed silica with a BET surface area of 300 m²/g, as a result of the addition of hydrophobic silica before the grinding, the grindometer value of 10 μm at a tamped density of 28 g/l and of 15 to 20 μm at a tamped density of 50 g/l are achieved.

The known fumed silicas have the disadvantage that they still have relatively high grindometer values and hence their contribution to increasing the viscosities and thixotropy is not optimal, and that the values worsen in the course of prolonged storage.

The technical object was therefore to provide fumed silanized silicas which have improved rheological properties and low grindometer values.

The technical object is achieved by a hydrophobic fumed silica which is obtained by grinding a fumed silica which, as a result of silanization, has trimethylsilyl groups fixed on the surface.

Preferably, the grinding is effected with a pin mill or an air-jet mill. This affords silicas which have a lower grindometer value than the unground starting material used, i.e. the unground silicas. The ground silica of the invention is therefore better and more rapidly dispersible, for example, in coating formulations.

In a preferred embodiment, the silica of the invention has a tamped density of 10 to 100 g/l, preferably of 15 to 65 g/l.

Fumed silicas are known from Winnacker-Kuchler Chemische Technologie [Chemical technology], volume 3 (1983), 4th edition, page 77, and Ullmanns Enzyklopädie der technischen Chemie, 4th edition (1982), volume 21, page 462.

In particular, fumed silicas are prepared by flame hydrolysis of evaporable silicon compounds, for example SiCl₄, or organic silicon compounds, such as trichloro-methylsilane.

The silicas in the context of the present invention are fumed silicas, the surface having been modified with at least one organic component. They are therefore referred to as surface-modified silicas. Modified fumed silicas (silicas prepared from fumed silicas) are understood to means silicas which can be prepared on the basis of fumed silicas according to DE 24 14 478. Surface modification is understood to mean the chemical and/or physical attachment of organic components to the surface of the silica particles. In other words, in the case of surface-modified silicas, at least part of the surface of at least some of the silica particles is covered with the surface modifiers. In the present case, the fumed silicas are silanized by reacting fumed silica with trimethylchlorosilane or trimethylsilanol or hexamethyldisilazane in a known manner, the trimethylsilyl groups being fixed on the surface of the fumed silica.

The silica of the invention does not tend to reagglomerate. The grindometer value of the silica of the invention is lower than that of the starting material and, in the case of a dispersion time of 30 min, is 12 μm or less, while the grindometer value of the unground silica is 15 μm. In the case of a dispersion time of 60 min, the grindometer value is below 10 μm.

In a further preferred embodiment, the silica of the invention has a specific BET surface area of 150 to 300 m²/g, preferably of 200 to 295 m²/g, more preferably of 195 to 290 m²/g.

It is also preferred that the silica of the invention has a mean particle size according to Cilas of 3.5 to 8.5. In a further preferred embodiment, the silica according to the present invention has a pH in the range of 5.0 to 8.0.

The silica of the invention preferably has an agglomerate strength of less than 25 mm, more preferably of less than 20 mm.

The invention further provides a process for producing the silica of the invention, the process being characterized by the step of grinding a fumed silica which, as a result of silanization, has trimethylsilyl groups fixed on the surface.

In a preferred process, the silica used has a BET surface area of 150 to 350 m²/g, preferably of 180 to 300 m²/g and a tamped density of 50 to 100 g/l, preferably of 55 to 65 g/l and more preferably of approx. 60 g/l.

In a further preferred embodiment of the preparation process according to the present invention, the silicas used have the following physicochemical characteristic data:

BET surface area m²/g: 150-300 Mean size of the primary 7 particles nm: pH: 5.0-9.0 carbon content % by weight: 0.1 to 10, preferably 2.0-4.0

In a further preferred preparation process, the silicas used also have the following physicochemical characteristic data:

Tamped density²⁾ g/l Approx. 60 Drying loss³⁾ (2 h at 105° C.) <=0.5 % by weight on departure from the manufacturer Ignition loss⁴⁾⁵⁾ (2 h at 1000° C.) 1.0-3.0 % by weight pH⁶⁾⁷⁾ 5.5-7.5 SiO₂ content⁸⁾ % by weight >=99.8 Al₂O₃ content⁸⁾ % by weight <=0.05 Fe₂O₃ content⁸⁾ % by weight <=0.01 TiO₂ content⁸⁾ % by weight <=0.03 HCl content⁸⁾⁹⁾ % by weight <=0.025 ¹⁾To DIN ISO 9277 ²⁾To DIN EN ISO 787-11, JIS K 5101/20 (unscreened) ³⁾To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23 ⁴⁾To DIN EN 3262-20, ASTM D 1208, JIS K 5101/24 ⁵⁾Based on the substance dried at 105° C. for 2 hours ⁶⁾To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26 ⁷⁾Water:methanol = 1:1 ⁸⁾Based on the substance calcined at 1000 C. for 2 hours ⁹⁾HCl content is part of the ignition loss

The silicas of the invention are used as thickeners or thixotropic agents in coating formulations.

The present invention therefore also provides coating formulations comprising a hydrophobic fumed silica which is obtained by grinding a fumed silica which, as a result of silanization, has trimethylsilyl groups fixed on the surface. The ground silica present has a lower grindometer value than the unground silica. The grindometer value may be less than 12 μm.

In a preferred embodiment, the tamped density of the silica is 10 to 100 g/l, preferably 15 to 65 g/l.

Coating formulations in the context of the present invention are coating formulations comprising at least one polymer component and/or a mixture of a plurality of physically or chemically crosslinking polymer components, at least one solvent and at least one surface-modified silica. The coating formulations of the invention are preferably 1-component coatings, 2-component coatings and UV coatings, especially polyurethane coatings, and most preferably clearcoats and matt coating formulations.

A clearcoat in the sense of the invention is a coating material which, applied on a substrate, forms a transparent coating with protective, decorative or specifically technical properties. In a coating system, the clearcoat protects, as the uppermost layer, the layers below it from mechanical damage and weathering influences. A clearcoat does not comprise any pigments. Especially in the case of clearcoats, the transparency of the coating, i.e. the visual impression of how clear and undistorted the surface of the material coated with the clearcoat can be seen through the coating after it has dried, is a measure of the quality of the coating. When the clearcoat is applied on a shiny black background, the blackness value M_(y) can be employed as a measure for the transparency of this coating.

In a preferred embodiment, the coating formulation at a reflectometer value of 85 to 90, preferably of 87 to 88, has a blackness value M_(y) of at least 280, preferably of at least 285.

It is also preferred that the coating formulation comprises 0.5 to 15% by weight of the silica.

In addition to the components mentioned, the coating formulations of the invention may also comprise further assistance and additives typically used in coatings, for example plasticizers, stabilizers, phase mediators, pigments, surfactants, desiccants, catalysts, initiators, photosensitizers, inhibitors, light stabilizers and preservatives.

Coating formulations of the invention may, as binders, comprise the resins customary in paints and coatings technology, as described, for example, in “Lackharze, Chemie, Eigenschaften and Anwendungen [Coating resins, chemistry, properties and applications], Eds. D. Stoye, W. Freitag, Hanser Verlag, Munich, Vienna 1996”. The contents of this publication are hereby incorporated explicitly into the content of the description of the present invention. Examples include the polymers and copolymers of (meth)acrylic acid and their esters, which optionally bear further functional groups, with further olefinically unsaturated compounds, for example styrene; polyetherpolyols, polyesterpolyols, polycarbonatepolyols, polyurethanepolyols and epoxy resins, and any desired mixtures of these polymers, and also fatty acid-modified “alkyd resins” prepared by polycondensation, as described in Ullmann, 3rd edition, volume 11, page 334 ff. The contents of this publication are hereby incorporated explicitly into the content of the description of the present invention.

Preference is given to using, as polymer components, organic compounds bearing hydroxyl groups, for example polyacrylatepolyols, polyesterpolyols, polycaprolactonepolyols, polyetherpolyols, polycarbonatepolyols, polyurethanepolyols and hydroxy-functional epoxy resins, and any mixtures of these polymers. The particularly preferred polymeric organic compounds used are aqueous or solvent-containing or solvent-free polyacrylatepolyols and polyesterpolyols and any mixtures thereof.

Suitable polyacrylatepolyols are copolymers of, inter alia, monomers having hydroxyl groups with other olefinically unsaturated monomers, for example esters of (meth)acrylic acid, styrene, [alpha]-methylstyrene, vinyltoluene, vinyl esters, maleic and fumaric mono- and dialkyl esters, [alpha]-olefins and further unsaturated oligomers and polymers.

In a further preferred embodiment, the coating formulation comprises 5.0 to 99.5% by weight of solids of a polymer component or of a mixture of two or more physically or chemically crosslinking polymer components and/or 0 to 99.5% by weight of a low molecular weight component which functions as a solvent or of a mixture of such low molecular weight components.

It is also preferred that the coating formulation comprises at least one binder selected from the group consisting of polymers and copolymers of (meth)acrylic acid and esters thereof, which optionally bear further functional groups, with further olefinically unsaturated compounds, for example styrene; polyetherpolyols, polyesterpolyols, polycarbonatepolyols, polyurethanepolyols, epoxy resins, and fatty acid-modified alkyd resins prepared by polycondensation.

The present invention is illustrated with reference to the examples which follow, which do not, however, restrict the scope of protection.

EXAMPLES

1. Grinding

To prepare the inventive examples, commercial AEROSIL® R812 or AEROSIL® R8125 (sack goods) were metered into the mill used with a metering balance and ground. The physicochemical characteristic data of the AEROSIL® R812 or AEROSIL® R8125 are listed in Table 1.

TABLE 1 Fumed silica used AEROSIL ® R812 AEROSIL ® R8125 Behaviour towards water Hydrophobic Hydrophobic Appearance White powder White powder BET surface area¹⁾ 230-290 195-245 m²/g Mean size of the primary 7 7 particles nm Tamped densities²⁾ g/l Approx. 60 Approx. 60 Drying loss³⁾ (2 h at 105° C.) <=0.5 <=0.5 % by weight on departure from the manufacture Ignition loss⁴⁾⁵⁾ (2 h at 1.0-2.5 1.5-3.0 1000° C.) % by weight Carbon content % by weight 2.0-3.0 3.0-4.0 pH⁶⁾⁷⁾ 5.5-7.5 5.5-7.5 SiO₂ content⁸⁾ % by weight >=99.8 >=99.8 Al₂O₃ content⁸⁾ & by weight <=0.05 <=0.05 Fe₂O₃ content⁸⁾ & by weight <=0.01 <=0.01 TiO₂ content⁸⁾ & by weight <=0.03 <=0.03 HCl content⁸⁾⁹⁾ & by weight <=0.025 <=0.025 ¹⁾To DIN ISO 9277 ²⁾To DIN EN ISO 787-11, JIS K 5101/20 (unscreened) ³⁾To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23 ⁴⁾To DIN EN 3262-20, ASTM D 1208, JIS K 5101/24 ⁵⁾Based on the substance dried at 105° C. for 2 hours ⁶⁾To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26 ⁷⁾Water:methanol = 1:1 ⁸⁾Based on the substance calcined at 1000 C. for 2 hours ⁹⁾HCl content is part of the ignition loss

For the tests, a pin mill (Alpine 160Z, rotor diameter 160 mm) of an air-jet mill (grinding space diameter: 240 mm, grinding space height: 35 mm) was used. The ground product was isolated with a bag filter (filter area: 3.6 m², filter material: nylon fabric). In further tests, the resulting ground product was packed into commercial sacks with a commercial bagging machine. In further tests, the sacks packed with ground product were levelled with a technically customary method suitable for this purpose before palleting. The levelled sacks were, as is commercially customary, palleted and subsequently stored over five weeks. The parameters of the production process are listed in Table 2.

TABLE 2 The table shows the parameters of the production of some examples of the silica of the inventions GA** GA** IA*** IA*** rate pressure rate pressure Metering Example Mill* [m³] [bar] [m³] [bar] [kg/h] Bagging Levelling Storage 1 AJ 27.5 3.5 15.9 3.7 10 No No No 2 AJ 27.5 3.5 15.9 3.7 10 Yes No No 3 AJ 27.5 3.5 15.9 3.7 10 Yes Yes No 4 AJ 27.5 3.5 15.9 3.7 10 Yes Yes Yes 5 AJ 11.5 1.0 6.8 1.2 10 No No No 6 AJ 11.5 1.0 6.8 1.2 10 Yes No No 7 AJ 11.5 1.0 6.8 1.2 10 Yes Yes No 8 AJ 11.5 1.0 6.8 1.2 10 Yes Yes Yes 9 PM — — — — 10 No No No 10 PM — — — — 10 Yes No No 11 PM — — — — 10 Yes Yes No 12 PM — — — — 10 Yes Yes Yes 13 PM — — — — 20 No No No 14 PM — — — — 20 Yes No No 15 PM — — — — 20 Yes Yes No 16 PM — — — — 20 Yes Yes Yes *AJ = air-jet mill; PM = pin mill, GA** = grinding air; IA*** = injector air 2. Determination of the Physicochemical Characteristic Data of the Ground Silicas 2.1 BET Surface Area

The BET surface area is determined to DIN ISO 9277.

2.2 Tamped Density

The tamped density was determined to DIN EN ISO 787-11. A defined amount of a sample which had not been screened beforehand is filled into a graduated glass cylinder and subjected to a fixed number of tamping operations by means of a tamping volumeter. During the tamping, the sample is compacted. As a result of the analysis carried out, the tamped density is obtained.

Fundamentals of the Tamped Density Determination:

The tamped density (formerly tamped volume) is equal to the quotient of the mass and the volume of a powder after tamping in a tamping volumeter under fixed conditions. According to DIN ISO 787/XI, the tamped density is reported in g/cm³. Owing to the very low tamped density of the oxides, however, the value is reported here in g/l. In addition, the drying and screening and the repetition of the tamping operation are dispensed with.

Equipment for Tamped Density Determination:

-   tamping volumeter -   measuring cylinder -   laboratory balance (readability 0.01 g)

Performance of the Tamped Density Determination:

200±10 ml of oxide are filled into the measuring cylinder of the tamping volumeter such that no cavities remain and the surface is horizontal. The mass of the sample introduced is determined accurately to 0.01 g. The measuring cylinder with the sample is placed into the measuring cylinder holder of the tamping volumeter and tamped 1250 times. The volume of the tamped oxide is read off accurately to 1 ml.

The Valuation of the Tamped Density Determination:

${{tamped}\mspace{14mu}{density}\mspace{14mu}\left( {g\text{/}l} \right)} = \frac{{Initial}\mspace{14mu}{weight}\mspace{14mu}{in}\mspace{14mu} g \times 1000}{{Volume}\mspace{14mu}{read}\mspace{14mu}{off}\mspace{14mu}{in}\mspace{14mu}{ml}}$ 2.3 pH

Reagents for pH Determination:

-   distilled or demineralized water, pH>5.5 -   methanol, p.a. -   buffer solutions pH 7.00, pH 4.66

Equipment for pH Determination:

-   laboratory balance (readability 0.1 g) -   beaker, 250 ml -   magnetic stirrer -   stirrer bar, length 4 cm -   combined pH electrode -   pH measuring instrument -   Dispensette, 100 ml

Procedure for Determining the pH:

the determination was effected to DIN EN ISO 787-9.

Calibration: before the pH measurement, the measuring instrument is calibrated with the buffer solutions. When a plurality of measurements are carried out in succession, a single calibration is sufficient.

4 g of oxide are converted to a paste with 48 g (61 ml) of methanol in a 250 ml beaker, and the suspension is diluted with 48 g (48 ml) of water and stirred with a magnetic stirrer in the presence of an immersed pH electrode (speed approx. 1000 min⁻¹).

After the stirrer has been switched off, the pH is read off after a wait time of one minute. The result is displayed with one decimal place.

Table 3 summarizes the physicochemical data of the silica of the invention and of the comparative example.

TABLE 3 Physicochemical data of the silicas of the invention Specific BET Tamped Median Agglomerate surface area density [μm] strength* Designation [m²/g] pH [g/l] (Cilas) [mm] Comparative 219 7.0 59 16.47 25 example Example 1 216 7.4 22 3.7 n.m. Example 2 216 7.5 65 3.69 16 Example 3 218 7.7 28 3.68 16 Example 4 218 6.7 51 3.74 16 Example 5 217 7.5 20 8.05 n.m. Example 6 217 7.5 59 8.11 13 Example 7 217 7.5 54 8.22 16 Example 8 217 7.1 62 7.35 17 Example 9 218 7.5 20 5.85 n.m. Example 10 217 7.5 56 6.32 12 Example 11 219 7.2 56 6.03 12 Example 12 218 7.1 48 5.93 16 Example 13 217 6.9 17 7.18 n.m. Example 14 218 7.4 40 7.0 12 Example 15 218 7.2 65 7.23 17 Example 16 218 7.0 55 6.66 16 *n.m. = not measurable 3. Performance Tests 3.1 Test Procedure

The patterns described in the examples were performance-tested in a 2K PU clearcoat based on acrylate/isocyanate in comparison to unground starting material. The raw materials used were as follows: Macrynal SM 510n CH: 130010625 (Surface Specialities), Desmodur N 75 MPA (Bayer)

Table 4 shows the formulation of the 2K PU clearcoat comprising the ground silicas of the invention

TABLE 4 Formulation of the 2K PU clearcoat Parts by wt. Millbase Macrynal SM 510n 60 LG 23.34 Butyl acetate 98% 8.48 AEROSIL ® 0.70 Letdown Millbase 32.52 Macrynal SM 510n 60 LG 33.34 Xyrene 3.92 Ethoxypropyl acetate 3.46 Butylglycol acetate 1.50 Butyl acetate 98% 3.93 Curing agent Desmodur N 75, 75% strength 21.33 Σ 100.00

Table 5 shows the steps for producing and testing the 2K PU clearcoat.

TABLE 5 Manufacture and testing of the 2K PU clearcoat Predispersion disperse 2.5 times the amount of millbase with a dissolver at 2500 rpm for 5 min Dispersion 45 min in a Skandex disperser 250 ml glass bottles with addition of 200 g of glass beads, grindometer determination after 30 and 60 min Letdown with an initial charge of the millbase, the letdown mixture (Macrynal with the remaining components) is added. The homogenization is effected with a paddle stirrer. Addition of the the curing agent Desmodur N 75 is curing agent added with stirring (1000 rpm). Subsequently, the mixture is homogenized for 1 min. Application spray application at 21 s DIN 4 mm on to metal sheets painted black (DT36) with a spraying machine Setting: 1 crosscoat at setting 3.8; dry layer thickness: approx. 40 μm Spray dilution: Xylene 50 Ethoxypropyl acetate 6 Butylglycol acetate 6 Butyl acetate 98% 38 Drying approx. 24 h at RT, then at 70° C. for conditions 2 h 20° the shine and the cloudiness are reflectometer assessed on coating films which have value, haze been applied to black metal sheets with a reflectometer from Byk Gardner Blackness value the blackness value is determined on M_(y) coating films which have been (assessment of applied to metal sheets painted transparency) black, with a D19C densitometer from Gretag Macbeth. The blackness value M_(y) is obtained by multiplying the value measured by one hundred Wave scan the profile is assessed by means of (profile) a wave-scan plus system from Byk-Gardner 3.2 Grindometer Value 3.2.1 Basics

The degree of dispersion determines the performance properties of the Aerosil-thickened liquid. The measurement of the grindometer value serves for assessment of the degree of dispersion. The grindometer value is understood to mean the interface layer thickness below which the spots or aggregates present become visible on the surface of the exposed sample.

The sample is exposed with a scraper in a groove whose depth at one end is twice as great as the diameter of the largest Aerosil grains and decreases constantly to 0 at the other end. On a scale which specifies the depth of the groove, the value of the depth in micrometers below which a relatively large number of Aerosil grains become visible as a result of spots or scratches on the surface of the binder system is read off. The value read off is the grindometer value of the present system.

3.2.2. Performance of the Grindometer Value Determination

The grindometer block is placed onto a flat, slip-resistant surface and wiped clean immediately before the test. The Aerosil dispersion, which must be free of air bubbles, is then applied at the lowest point in the groove such that it flows away a little above the edge of the groove. The scraper is then gripped with both hands and placed onto the end of the groove in which the dispersion is present perpendicularly to the grindometer block and at right angles to its longitudinal edges with gentle pressure. The dispersion is then exposed by slow, homogenous pulling of the scraper over the block in the groove. No later than 3 seconds after the exposure of the dispersion, the grindometer value is read off.

In the determination, the surface of the exposed dispersion (at right angles to the channel) is viewed obliquely from above at an angle of 20-30° (to the surface). The block is held to the light such that the surface structure of the exposed dispersion is readily discernible.

On the scale, the grindometer value read off is the value in micrometers below which a relatively large number of Aerosil grains become visible as spots or scratches on the surface. Individual spots or scratches which appear coincidentally are not taken into account.

The granularity is assessed at least twice, and in each case on a newly exposed dispersion.

3.2.3 Evaluation

The measurements are used to form the arithmetic means. There exists the following relationship between the grindometer value in micrometers and the Hegmann units and FSTP units based on the target system: B=8−0.079 A C=10−0.098 A=1.25 B

In the equations:

-   A=grindometer value in micrometers -   B=grindometer value in Hegmann units -   C=grindometer value in FSTP units

Table 6 shows the dispersibility of the silicas of the invention with reference to the grindometer values measured.

TABLE 6 Dispersibility Grindometer value (μm) of the millbases 30 min Starting material/ 15 Comparative example Example 1 12 Example 2 10 Example 3 10 Example 4 10 Example 5 12 Example 6 12 Example 7 12 Example 8 12 Example 9 12 Example 10 12 Example 11 12 Example 12 12 Example 13 12 Example 14 12 Example 15 12 Example 16 12 3.3 Optical Properties 3.3.1 Determination of the 20° Reflectometer Value, Haze

In order to assess any influence on the shine and on the haze through the presence of the silica, the 20° reflectometer value is measured. The reflectometer value is thus an important criterion for the characterization of coating films.

3.3.2 Determination of the Transparency as the Blackness Value M_(y)

The blackness value M_(y) is determined on coating films which have been applied to metal sheets painted black, using a D19C densitometer from Gretag Macbeth. The value M_(y) gives a statement about the colour depth and transparency of the clearcoat. The higher this value is, the more transparent is the coating. Simultaneously, the colour depth increases.

Table 7 summarizes the results of the optical properties of the 2K PU clearcoats comprising the silicas of the invention.

TABLE 7 Optical properties 20° reflectometer Blackness Wave scan value Haze value My L S Starting 87.8 11 286 23 28 material Example 4 87.8 10 288 22 28 Example 8 87.8 10 285 24 29 Example 12 87.7 11 286 29 28 Example 16 87.8 10 287 26 33

The data of the ground products show lower grindometer values with virtually equal specific surface areas. Surprisingly, the lower grindometer values are maintained in spite of compaction, recognizable by the tamped density, as a result of bagging or bagging/levelling and bagging/levelling/storage. In some cases, the tamped densities are even above that of the oxide used, i.e. the oxides of the invention possess, in spite of equal or even higher compaction, lower grindometer values. The measurements which were carried out on the coating films comprising the silicas of the invention show that the quality criteria of shine, haze and transparency are satisfied with simultaneously improved properties with regard to the rheology and dispersibility of the silicas in the coating formulations. 

1. A ground hydrophobic fumed silica comprising a fumed silica which has trimethylsilyl groups present on the surface, wherein the ground hydrophobic fumed silica has a tamped density of from 10 to 65 g/l; a specific BET surface area of from 210 to 225 m²/g; and a grindometer value of 12 μm or less for a dispersion time of 30 minutes or a value of less than 10 μm for a dispersion time of 60 minutes.
 2. The silica according to claim 1, wherein the ground hydrophobic fumed silica is obtained by a process comprising grinding fumed silica with a pin mill or an air-jet mill.
 3. The silica according to claim 1, which has a mean particle size of from 3.5 to 8.5 μm.
 4. The silica according to claim 1, wherein the silica has a pH in the range of 6.5 to 8.0.
 5. The silica according to claim 1, which has an agglomerate strength of less than 25 mm.
 6. A process for producing the fumed silanized silica according to claim 1, comprising grinding a fumed silica which, as a result of silanization, has trimethylsilyl groups fixed on the surface.
 7. The silica according to claim 1, which has a mean particle size of from 3.5 to 8.22 μm.
 8. The silica according to claim 1, which has a tamped density of from 17 to 65 g/l.
 9. The silica according to claim 1, which has a specific BET surface area of from 216 to 219 m²/g.
 10. The silica according to claim 1, which has a tamped density of from 17 to 65 g/l, and a specific BET surface area of from 216 to 219 m²/g.
 11. The silica according to claim 1, which has an agglomerate strength of less than 20 mm. 